Synthesis, Structure, and Reactivity of Dinuclear Nickel Amino-Thiophenolate Complexes Bearing Bridging VO2(OH)2– and VO2(OR)2– Coligands

A series of novel mixed ligand dinickel complexes of the type [NiII2L­(μ-L′)]+, where L′ is a tetrahedral oxo-alkoxo vanadate (L′ = [O2VV(OR)2]−, R = H or alkyl) and L a macrocyclic N6S2 supporting ligand, have been prepared, and their esterification reactivity has been studied. The orthovanadate complex [Ni2L­(μ-O2V­(OH)2)]+ (2), prepared by reaction between [Ni2L­(μ-Cl)]­ClO4 with Na3VO4 and a phase transfer reagent in CH3CN, reacts smoothly with MeOH and EtOH forming the vanadate diesters [Ni2L­(μ-O2V­(OMe)2)]+ (3) and [Ni2L­(μ-O2V­(OEt)2)]+ (4). The dialkyl orthovanadate esters in 3 and 4 are readily transesterified with mono- and difunctional alcohols. Complex 3 can also be generated from 4 by transesterification with MeOH. Complexes 3 and 4 react with diols (ethylene glycol, propylene glycol and diethylene glycol) as well to afford the complexes [Ni2L­(μ-O2V­(OH)­(OCH2CH2OH))]+ (5), [Ni2L­(μ-O2V­(OCH2)2CH2)]+ (6), and [Ni2L­(μ-O2V­(OCH2CH2)2O)] (7). The crystal structures of the tetraphenylborate salts of complexes 3–7 reveal in each case four-coordinate O2VV(OR)2– groups bonded in a μ1,3-bridging mode to generate trinuclear complexes with a central N3Ni­(μ-S)2(μ1,3-O2V­(OR)2)­NiN3 core. The stabilization of the four-coordinate VVO2(OR)2– moieties is a consequence of both the two-point coordinative fixation to and the steric protection of the bowl-shape binding pocket of the [Ni2L]2+ fragment. Cyclic voltammetry experiments reveal that the encapsulated vanadate esters are not reduced in a potential window of −2.0 to +2.5 V vs SCE. The spins of the nickel­(II) (Si = 1 ions) in 3 are weakly ferromagnetically coupled (J = +23 cm–1, (H = −2JS1S2)) to produce an S = 2 ground state.